The present invention relates to a dynamic gain flattening filter, and more specifically, to a dynamic gain flattening filter for wavelength multiplexed optical fiber communication links and networks.
Dynamic gain equalizers (DGE) or dynamic gain flattening filters (DGFF) are frequently used in wavelength division multiplexing (WDM) fiber communication networks to equalize non-uniform signal intensity levels over a wide spectral bandwidth, which otherwise lead to transmission errors.
In particular, DGFFs are useful in WDM fiber communication networks employing erbium-doped fiber amplifiers (EDFAs) for simultaneously amplifying many signals at different wavelengths. In these networks, gain is dependent upon signal wavelengthxe2x80x94resulting in a characteristic gain curve or spectrum. In a typical EDFA, the gain spectrum window is in the order of 30 nm (1530-1560 nm).
A challenge in DGFF design is to achieve a continuity of gain curve without ripple or discontinuities over the entire spectral window, and in particular, over a bandwidth occupied by a single channel, wherein channel bandwidths may overlap. This challenge has not been met by conventional channel power equalizers, which typically exhibit a stepped or rippled spectral response.
In U.S. Pat. No. 5,943,158 to Ford et al. there is disclosed a micro-mechanical modulator capable of producing a continuous gain spectrum. The micro-mechanical modulator is based on a mechanical anti-reflection switch (MARS) that provides a continuous, uniform optical surface. A diffraction grating disperses input light by wavelength, which is focussed by a lens into different light spots on the continuous, uniform surface. This continuous membrane is physically moved under an electrostatic force provided by a plurality of electrodes to selectively attenuate the input light by changing the spacing between the membrane and an underlying reflective surface, which changes the reflectivity of the modulator.
Although the modulator disclosed by Ford et al. provides reasonably smooth flattening, it is limited by the mechanical properties of the continuous membrane, which results in a coupling between the controls exercised by nearby electrodes, and limits the achievable spatial resolution. Thus, it does not provide the flexibility and advantages associated with the discrete arrays available for use in DGFF devices.
It is an object of the instant invention to provide a DGFF that offers smooth gain flattening and that uses a discrete array of elements.
The instant invention provides a dynamic gain flattening filter that offers a smooth spectral response. Conveniently and advantageously, the dynamic gain flattening filter of the instant invention includes currently manufactured discrete arrays for selectively attenuating a multiplexed optical signal. A dispersive element, such as diffraction grating, disperses an input beam of light into a plurality of monochromatic sub-beams. Each monochromatic sub-beam is incident on more than one element of the discrete array. Since each sub-beam of light is incident on a plurality of elements in the array, a smooth spectral response is provided at an output of the device. This is in contrast to channel equalizers that limit each sub-beam of light to be incident on a single element, which tends to provide a rippled or stepped spectral response.
In accordance with the instant invention there is provided a dynamic gain flattening filter comprising a first port for launching a beam of light comprising multiple wavelengths, a dispersive element for dispersing the beam of light into a plurality of sub-beams of light, and, a discrete array of controllable elements for receiving the plurality of sub-beams of light, wherein the dynamic gain flattening filter is designed such that each sub-beam of light is incident on more than one element of the discrete array for selective attenuation before being recombined and redirected to one of the first and a second port.
In accordance with the instant invention there is provided a dynamic gain flattening filter comprising a first port for launching a beam of light comprising multiple wavelengths, a dispersive element optically coupled to the first port for spatially dispersing the beam of light into a plurality of sub-beams, and a discrete array of controllable elements optically coupled to the dispersive element for receiving the plurality of sub-beams, wherein each received sub-beam is incident on a predetermined number of elements for providing a continuous spectral response at one of the first and a second port.
In accordance with the instant invention there is further provided a method of attenuating a multiplexed optical signal comprising launching a beam of light comprising multiple wavelengths from a first port, spatially dispersing the beam of light into a plurality of sub-beams, focussing the plurality of sub-beams onto a focal plane, receiving the plurality of sub-beams at a discrete array of controllable elements, each sub-beam incident on more than one element, selectively attenuating each sub-beam with the more than one element, and recombining the selectively attenuated plurality of sub-beams into a multiplexed beam of light and transmitting it to one of the first and a second port.
In one embodiment, the first port corresponds to the second port of a 3 port optical circulator. In another embodiment, the second port is spatially separated from the first port, and the multiplexed beam is redirect to the second port via a mirror disposed between the dispersive element and the first port. Of course other embodiments are also possible.